The 7750 can be limited by it's lack of a PCIe power connector on the 800MHz stock models if the GPU isn't good at clocking without high voltage. Also, you should try starting off with a smaller overclock first. You're starting off with a fairly large 12.5% jump in frequency when you shouldn't increase more than 5% or so at a time.
How well a card overclocks depends on the individual card. I can't tell you how well your card overclocks because I don't have it to test it. The only way to figure out exactly how well it does is to test how well it overclocks AFAIK.